Nagssugtoqidian mobile belt in West Greenland

The rocks forming the northern part of the Archaean block in West Greenland are cut by dense swarms of dolerite dykes. Towards the north the dykes, together with their country rocks, arc progressively deformed and metamorphosed resulting in a reorientation and parallelisation of dykes and country rock structures. These changes were the basis on which Ramberg (1949) distinguished a Nagssugtoqidian complex from a pre-Nagssugtoqidian (Archaean) complex in West Greenland. Similar observations by Bridgwater & Gormsen (1968) in South-East Greenland made it possible to correlate the Nagssugtoqidian mobile belt from West to East Greenland. It forms a c. 300 km wide belt characterised by a pronounced regional fabric oriented parallel to the boundary with the Archaean block (fig. 70). In West Greenland this regional fabric is cut to the north by the wrench fault zone which separates the Nagssugtoqidian from the Rinkian mobile belt. Although K-Ar dating of Nagssugtoqidian rocks gives ages within the range 1740-1650 m.y. (Larsen & Møller, 1968), U-Pb dating of zircons (Chessex et al., 1973) suggests that the main phase of Nagssugtoqidian deformation and metamorphism is much older and probably took place at the beginning of the Proterozoic or at the end of the Archaean (fig. 71).